Thousands of Ukrainian opposition activists and supporters have rallied in Kyiv, demanding the release of jailed former Prime Minister Yulia Tymoshenko.

The protesters marched through the city center and held a mass gathering on Independence Square, where they called for Tymoshenko's immediate release and the resignation of President Viktor Yanukovych.

They also demanded a thorough and transparent investigation into the alleged beating of Tymoshenko during her forced transfer from a prison to a clinic last week for treatment of debilitating back pain.

"We have just asked Viktor Yanukovych personally to come here and explain what he is doing in Ukraine and why he is illegally torturing and killing Tymoshenko," Yuriy Odarchenko, a member of Tymoshenko's parliamentary faction, told journalists at the rally.

PHOTO GALLERY: Pictures released by the Ukrainian ombudsman that purportedly show bruises on Yulia Tymoshenko's body

Pictures Of Tymoshenko's Bruises

The demonstrators were holding banners of opposition parties and movements that organized the event, dubbed “Ukraine Without Repression.”

According to the press service of Tymoshenko's Batkivshchyna ("Fatherland") Party, 25 people are now on hunger strike in Kyiv, Lviv, and other cities in solidarity with the former prime minister, who launched her own hunger strike a week ago to protest alleged abuse.

The events on April 27 come a day after Yanukovych ordered an investigation into Tymoshenko's alleged beating.

The former prime minister was sentenced in October to seven years in prison on charges of abuse of power. She says she is the victim of political persecution.

Her husband, Oleksandr Tymoshenko, told the Reuters news agency that Yanukovych's government is trying put his wife under unbearable pressure.

"The current government has focused its entire repressive machine on trying to achieve one goal: to destroy my wife, Yulia Tymoshenko, physically and politically," he said. "That is why I have appealed to the leaders of several countries so that they can judge the real situation in Ukraine."

Germany: 'Don't Play For Time'

Also on April 27 in Berlin, German doctors who met and examined Tymoshenko earlier this month said they were skeptical that she can be successfully treated for her back condition in Ukraine.

Karl Einhaeupl of the Charite Hospital displays two magnetic resonance tomography images of the spine of Yulia Tymoshenko that show a spinal disc herniation during a news conference in Berlin on April 27.

Karl Max Einhaeupl, the head of Berlin's Charite University Hospital, said it was "unlikely" that the treatment in Kharkiv Hospital would succeed because the country did not have the expertise.

Einhaeupl said that Tymoshenko, who went on a hunger strike a week ago, was refusing treatment because "she does not trust the Ukrainian medical system," for fear of being deliberately infected.

"We are ready to receive Mrs. Tymoshenko here," he said. "The Ukrainian authorities would have to be ready to suspend the prison sentence, at least for a limited period of time to take her to Germany."

Meanwhile, German Chancellor Angela Merkel's spokesman, Steffen Seibert, urged Ukraine not to "play for time" in the case of Tymoshenko.

Seibert said further developments would have an effect on whether Merkel will travel to Ukraine in June to watch the German team play in the Euro 2012 soccer championship.

There have been calls for Germany to boycott the tournament in Ukraine over the case of Tymoshenko. Ukraine is co-hosting the event with Poland.

German President Joachim Gauck has already canceled a planned visit to Ukraine next month amid concerns over Tymoshenko's treatment.

The EU's foreign-policy chief, Catherine Ashton, has said she is "deeply preoccupied" by the case and has urged Kyiv to allow the EU ambassador to visit Tymoshenko along with independent medical experts.